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Top Forums Shell Programming and Scripting IF statement with square brackets Post 302983621 by mad man on Friday 14th of October 2016 02:41:49 AM
Old 10-14-2016
IF statement with square brackets

Hi All,

Hope you all are doing good. Yesterday in my project i came across a scenario which i can not guess why it was working in one region and why it was not in another region. Please find my issue below.

I am using AIX version 6.0 of UNIX in my project, in shell scripting i have the following code to check if a file exists in a directory or not.

Code:
       
if [ -f $com_dir/$dup_file_name_chk ] || 
[ -f $save_dir/$dup_file_name_chk ]
then
.
.
.
fi

I tested this code in the development region every thing went fine and moved the code to QA there also testing went fine. After 2 successful tests the code was moved to UAT region, my onsite manager tested it in UAT and found this is not working, the file existence was not detected there and testing failed.

After this i doubted this if condition and made it like below.

Code:
       
if [[ -f $com_dir/$dup_file_name_chk ]] || 
[[ -f $save_dir/$dup_file_name_chk ]]
then
.
.
.
fi

Now the same was tested in dev, QA & UAT regions. Now this file existence check was working in UAT region.

Can anyone explain what made this '[[' double brackets to work in the UAT region instead of '[' single bracket. What is the difference between them?
Also why regions differ(single brackets working in dev & QA not in UAT).
 

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SMLISTSH(1)							  ICI executables						       SMLISTSH(1)

NAME
smlistsh - shared-memory linked list test shell SYNOPSIS
smlistsh partition_size DESCRIPTION
smlistsh attaches to a region of system memory (allocating it if necessary, and placing it under PSM management as necessary) and offers the user an interactive "shell" for testing various shared-memory linked list management functions. smlistsh prints a prompt string (": ") to stdout, accepts a command from stdin, executes the command (possibly printing a diagnostic message), then prints another prompt string and so on. The following commands are supported: h The help command. Causes smlistsh to print a summary of available commands. Same effect as the ? command. ? Another help command. Causes smlistsh to print a summary of available commands. Same effect as the h command. k The key command. Computes and prints an unused shared-memory key, for possible use in attaching to a shared-memory region. + key_value size The attach command. Attaches smlistsh to a region of shared memory. key_value identifies an existing shared-memory region, in the event that you want to attach to an existing shared-memory region (possibly created by another smlistsh process running on the same computer). To create and attach to a new shared-memory region that other processes can attach to, use a key_value as returned by the key command and supply the size of the new region. If you want to create and attach to a new shared-memory region that is for strictly private use, use -1 as key and supply the size of the new region. - The detach command. Detaches smlistsh from the region of shared memory it is currently using, but does not free any memory. n The new command. Creates a new shared-memory list to operate on, within the currently attached shared-memory region. Prints the address of the list. s list_address The share command. Selects an existing shared-memory list to operate on, within the currently attached shared-memory region. a element_value The append command. Appends a new list element, containing element_value, to the list on which smlistsh is currently operating. p element_value The prepend command. Prepends a new list element, containing element_value, to the list on which smlistsh is currently operating. w The walk command. Prints the addresses and contents of all elements of the list on which smlistsh is currently operating. f element_value The find command. Finds the list element that contains element_value, within the list on which smlistsh is currently operating, and prints the address of that list element. d element_address The delete command. Deletes the list element located at element_address. r The report command. Prints a partition usage report, as per psm_report(3). q The quit command. Detaches smlistsh from the region of shared memory it is currently using (without freeing any memory) and terminates smlistsh. EXIT STATUS
0 smlistsh has terminated. FILES
No configuration files are needed. ENVIRONMENT
No environment variables apply. DIAGNOSTICS
No diagnostics apply. BUGS
Report bugs to <ion-bugs@korgano.eecs.ohiou.edu> SEE ALSO
smlist(3) perl v5.14.2 2012-05-25 SMLISTSH(1)
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